The role of mid-band gap defect levels in persistent photoconductivity in RF sputtered SnO₂ thin films

Persistent photoconductivity (PPC) is observed in RF sputtered SnO₂ thin films under illumination with a range of below band gap peak energies from 2.18 eV to 3.18 eV. This study provides new insights into the PC effect in SnO₂ films. Previous studies have confined excitation exclusively to monochromatic near UV illumination or to spectral response that did not follow growth and decay patterns over time. We have found that over the range of energies used for our investigation, the photoconductivity increases up to several orders of magnitude and the level of induced photocurrent rises non-linearly with incident photon energy. The patterns of the response and the transient decay of photocurrent as a function of incident photon energy under vacuum and at atmospheric pressure at room temperature suggest that more than one mechanism accounts for PPC. These results point strongly to a mechanism that is not solely dependent on oxygen adsorption/desorption.